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<H3>region command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>region ID style args keyword value ... 
</PRE>
<UL><LI>ID = user-assigned name for the region 

<LI>style = <I>block</I> or <I>cylinder</I> or <I>plane</I> or <I>sphere</I> or <I>union</I> or <I>intersect</I> 

<PRE>  <I>block</I> args = xlo xhi ylo yhi zlo zhi
    xlo,xhi,ylo,yhi,zlo,zhi = bounds of block in all dimensions (distance units)
  <I>cylinder</I> args = dim c1 c2 radius lo hi
    dim = <I>x</I> or <I>y</I> or <I>z</I> = axis of cylinder
    c1,c2 = coords of cylinder axis in other 2 dimensions (distance units)
    radius = cylinder radius (distance units)
    lo,hi = bounds of cylinder in dim (distance units)
  <I>plane</I> args = px py pz nx ny nz
    px,py,pz = point on the plane (distance units)
    nx,ny,nz = direction normal to plane (distance units)
  <I>sphere</I> args = x y z radius
    x,y,z = center of sphere (distance units)
    radius = radius of sphere (distance units)
  <I>union</I> args = N reg-ID1 reg-ID2 ...
    N = # of regions to follow, must be 2 or greater
    reg-ID1,reg-ID2, ... = IDs of regions to join together
  <I>intersect</I> args = N reg-ID1 reg-ID2 ...
    N = # of regions to follow, must be 2 or greater
    reg-ID1,reg-ID2, ... = IDs of regions to intersect 
</PRE>
<LI>zero or more keyword/value pairs may be appended 

<LI>keyword = <I>side</I> 

<PRE>  <I>side</I> value = <I>in</I> or <I>out</I>
    <I>in</I> = the region is inside the specified geometry
    <I>out</I> = the region is outside the specified geometry 
</PRE>

</UL>
<P><B>Examples:</B>
</P>
<PRE>region 1 block -3.0 5.0 INF 10.0 INF INF
region 2 sphere 0.0 0.0 0.0 5 side out
region void cylinder y 2 3 5 -5.0 INF
region outside union 4 side1 side2 side3 side4 
</PRE>
<P><B>Description:</B>
</P>
<P>This command defines a geometric region of space.  Various other
commands use regions.  See the <A HREF = "group.html">group grid</A>, <A HREF = "group.html">group
surf</A>, and <A HREF = "dump_modify.html">dump_modify</A> commands for
examples.
</P>
<P>Commands which use regions typically test whether a point is contained
in the region or not.  For this purpose, coordinates exactly on the
region boundary are considered to be interior to the region.  This
means, for example, for a spherical region, a point on the sphere
surface would be part of the region if the sphere were defined with
the <I>side in</I> keyword, but would not be part of the region if it were
defined using the <I>side out</I> keyword.  See more details on the <I>side</I>
keyword below.
</P>
<P>The lo/hi values for the <I>block</I> or <I>cylinder</I> styles can be specified
as INF which means a large negative or positive number (1.0e20).
</P>
<P>For style <I>cylinder</I>, the c1,c2 params are coordinates in the 2 other
dimensions besides the cylinder axis dimension.  For dim = x, c1/c2 =
y/z; for dim = y, c1/c2 = x/z; for dim = z, c1/c2 = x/y.  Thus the
third example above specifies a cylinder with its axis in the
y-direction located at x = 2.0 and z = 3.0, with a radius of 5.0, and
extending in the y-direction from -5.0 to infinity.
</P>
<P>The <I>union</I> style creates a region consisting of the volume of all the
listed regions combined.  The <I>intersect</I> style creates a region
consisting of the volume that is common to all the listed regions.
</P>
<P>IMPORTANT NOTE: Regions in SPARTA are always 3d geometric objects,
regardless of whether the <A HREF = "dimension.html">dimension</A> of the simulation
2d or 3d.  Thus when using regions in a 2d simulation, for example,
you should be careful to define the region so that its intersection
with the 2d x-y plane of the simulation has the 2d geometric extent
you want.
</P>
<P>The <I>side</I> keyword determines whether the region is considered to be
inside or outside of the specified geometry.  Using this keyword in
conjunction with <I>union</I> and <I>intersect</I> regions, complex geometries
can be built up.  For example, if the interior of two spheres were
each defined as regions, and a <I>union</I> style with <I>side</I> = out was
constructed listing the region-IDs of the 2 spheres, the resulting
region would be all the volume in the simulation box that was outside
both of the spheres.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "dump_modify.html">dump_modify</A>
</P>
<P><B>Default:</B>
</P>
<P>The option default is side = in.
</P>
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